1. Field of the Invention
The present invention relates to a method of making a grain oriented silicon steel sheet suitable for use as an iron core for transformers or other electrical appliances. More particularly, the present invention pertains to a method of effectively manufacturing a grain oriented silicon steel sheet which exhibits excellent coating properties and which has reduced or no core loss as a result of stress-relieving annealing.
2. Description of the Related Art
Important properties of grain oriented silicon steel sheets include the magnetic properties of the steel sheet and the properties of the coating on the surface of the steel sheet, such as the insulation properties required when the steel sheets are laid on top of one another to manufacture an iron core. Also important are the peeling resistance properties required during manufacture. To improve the properties of the coating on the steel sheet, it is essential to improve the adhesion of a forsterite film generated during finish annealing.
It has been proposed to add a Ti compound, such as TiO.sub.2, to improve the forsterite film. This proposal suggested adding the TiO.sub.2 to the MgO, which is the main component of the annealing separating agent coated on the surface of the steel sheet prior to finish annealing. For example, Japanese Patent Publication No. 51-12451 discloses the technique of improving the uniformity and adhesion of a forsterite film by adding 2 to 40 parts by weight of Ti compound per 100 parts by weight of the Mg compound. Japanese Patent Publication No. 49-29409 describes the technique of improving the uniformity and adhesion of the forsterite film by adding 2 to 20 parts by weight of TiO.sub.2 per 100 parts by weight of heavy low-active fine grains of MgO. From these disclosures are developed various other techniques: for example, Japanese Patent Laid-Open No. 50-145315 discloses eliminating a sunspot-like attached material made up of a Ti compound by using pulverized TiO.sub.2 in the annealing separating agent. Japanese Patent Laid-Open No. 54-128928 discloses increasing the tension of the forsterite film by mixing TiO.sub.2 and SiO.sub. 2 and a boric compound with MgO. Japanese patent Laid-Open No. 1-168817 discloses the technique of improving the core loss by mixing TiO.sub.2, antimony sulfate and manganese nitride or ferromanganese nitride with MgO.
Although adding a Ti compound to the annealing separating agent may be effective to improve some properties of the coating, they strongly tend to increase core loss experienced as a result of stress-relieving annealing. This problem is described in Japanese patent Laid-Open No. 2-93021.
Many of the transformer iron cores made of a grain oriented silicon steel sheet are small core type iron cores called coiled cores. Since a stress is generated in such a coiled core when the coil is subjected to a mechanical external force during the deforming process in manufacture, and hence the magnetic properties thereof deteriorate, stress-relieving annealing must be conducted at about 800.degree. C. to eliminate the stress. However, if a Ti compound is present in the annealing separating agent, a carbide of Ti or a selenide or sulfide of Ti is precipitated in the portion of the surface of the ferrite to which the processing stress is applied during stress-relieving annealing. Consequently, the movement of the magnetic domain wall is partially prevented and the core loss thus increases. Thus a steel sheet which generates less core loss, even when stress-relieving annealing is conducted, has long been desired for use in coiled cores.
To prevent the increased core loss which is caused by application of stress-relieving annealing to a silicon steel sheet having Ti in an annealing separating agent, it has been proposed in Japanese Patent Laid-Open No. 2-93021 to decrease the amount of precipitated carbide of Ti. This is proposed to be done by decreasing to 0.0015 weight percent or less the amount of carbon in the silicon steel sheet which is to be subjected to finish annealing. However, when using this technique it is difficult as a practical matter to restrict absorption of carbon dioxide into MgO, and it is essentially impossible to decrease the amount of sulfide or selenide of Ti. It is thus impossible to substantially restrict the core loss caused by stress-relieving annealing.